1. Field of the Invention
The present invention relates to a technique for manufacturing a conductive resin composition having a low electrical resistance, in which technique a resin and a metal (lead-free solder) in a semi-melted state are kneaded to cause particles of the metal to disperse finely within the resin and to come into connection with one another in order to reduce the electrical resistance. The semi-melted state may be attained artificially through addition of unmeltable metal powder into a completely melted metal.
2. Description of the Related Art
Conventionally, conductive plastics have been manufactured through incorporation of metal fibers into a resin. Further, in order to improve the reliability of the conductive plastics, a metal having a low melting point has been added into the metal fibers to reduce the contact resistance. Such conductive plastics have been utilized as highly conductive plastics in various applications, such as electromagnetic wave shields.
However, since the conventional conductive plastics have a high volumetric resistivity of 10xe2x88x923 xcexa9xc2x7cm or greater, they generate heat when electricity flows therethrough, resulting in melting of the conductive plastics and increased resistance. Therefore, the conventional conductive plastics cannot be used for transmitting electricity.
Further, when the metal filler content is increased to reduce the volumetric resistivity, the moldability deteriorates, rendering injection molding impossible. In addition, when a solder containing leadxe2x80x94which is hazardous materialxe2x80x94is used, waste conductive plastics cause an environmental problem.
There has also been utilized a technique called MID (Molded Interconnect Device). In this technique, injection molding is conducted two times, and in the first injection molding, there is used a resin that has been subjected to a process for facilitating plating thereon. The thus-injected molded member is plated to form a metallic film on the surface thereof.
However, this technique requires a plating process. In addition, since electrically conductive circuits are formed through plating, the circuits cannot be formed within the resin.
As described above, in order to improve the conductivity of the conductive plastics, the metallic component must be mixed into the resin in a larger amount. However, when a fibrous material is mixed in the resin in a large amount, the moldability of the resin deteriorates, resulting in clogging of a nozzle, or impossibility of injection molding into a narrow-width shape. In addition, when a large amount of metal powder is mixed in the resin to establish connection between particles of the metal to thereby attain conductivity, the metal powder must be mixed in an amount of 60 vol. % or greater. In this case as well, the moldability deteriorates, and the attained conductivity is low. Further, since there is a difference between the resin and the metallic component in terms of thermal expansion, when the temperature increases, the number of contacts between the metal particles decreases due to expansion of the resin, resulting in deterioration in conductivity.
Moreover, there has been developed a conductive plastic to which is added a small amount of eutectic crystal solderxe2x80x94which is a metal having a low melting point. However, since the eutectic crystal solder contains lead, waste of the conductive plastic adversely affects the environment.
Also, the above-described MID technique also has the drawbacks of necessity of a plating process and the impossibility of forming conductive circuits within resin, the latter stemming from the fact that the resin itself does not have conductivity.
An object of the present invention is to solve the above-mentioned problems and to provide a super highly conductive plastic whose conductivity does not decrease in various environments and which is therefore highly reliable.
Another object of the present invention is to provide conductive circuits formed of the conductive plastic.
Still another object of the present invention is to provide a method of forming conductive circuits through use of the conductive plastic.
To achieve the first object, the present invention provides a conductive plastic formed of a conductive resin composition which comprises a thermoplastic resin, a lead-free solder that melts during plasticization, and metal powder or a mixture of metal powder and metal short fibers that assists the fine dispersion of particles of the lead-free solder within the thermoplastic resin.
Preferably, particles of the lead-free solder are dispersed such that the particles of the lead-free solder are in unbroken connection throughout the thermoplastic resin.
Preferably, the conductive resin composition has a volumetric resistivity of 10xe2x88x923 xcexa9xc2x7cm or less.
To achieve the second object, the present invention provides a conductive circuit including a wiring path that is formed in an insulating member through injection molding of a conductive resin composition which comprises a thermoplastic resin, a lead-free solder that melts during plasticization, and metal powder or a mixture of metal powder and metal short fibers that assists the fine dispersion of particles of the lead-free solder within the thermoplastic resin.
The wiring path may be formed for establishing connection between parts, for connection with a mounted component, or for connection with an electronic part.
To achieve the third object, the present invention provides a method of forming a conductive circuit in which a wiring path is formed in an insulating member through injection molding of a conductive resin composition which comprises a thermoplastic resin, a lead-free solder that melts during plasticization, and metal powder or a mixture of metal powder and metal short fibers that assists the fine dispersion of particles of the lead-free solder within the thermoplastic resin.
The wiring path may be formed for establishing connection between parts, for connection with a mounted component, or for connection with an electronic part.
The present invention also provides a method of forming a conductive circuit, the method comprising the steps of forming a frame having a space corresponding to a predetermined designed wiring path; and injecting into the space of the frame a conductive resin composition which comprises a thermoplastic resin, a lead-free solder that melts during plasticization, and metal powder or a mixture of metal powder and metal short fibers that assists the fine dispersion of particles of the lead-free solder within the thermoplastic resin, so that a wiring path is formed in the frame.
Preferably, a part is mounted on the frame in advance.
More preferably, spaces for receiving the leads of the part are formed in the frame, and the wiring paths are formed in the spaces.
Alternatively, a plurality of frames are prepared, spaces for receiving the leads of the part are formed in each of the frames, the plurality of frames are joined together such that the spaces of each frame correspond to the spaces of an adjacent frame, and the wiring paths are formed in the spaces.